docs: def get_etexts(feature_name, value):
      """Looks up all the texts that have meta-data matching some criterion.
  
      Arguments:
          feature_name (str): The meta-data on which to select the texts.
          value (str): The value of the meta-data on which to filter the texts.
  
      Returns:
          frozenset: The set of all the Project Gutenberg text identifiers that
              match the provided query.
  
      Raises:
          UnsupportedFeature: If there is no MetadataExtractor registered that
              can extract meta-data for the given feature name.
  
      """
      matching_etexts = MetadataExtractor.get(feature_name).get_etexts(value)
      return frozenset(matching_etexts)